(1) Field of the Invention
The present invention relates to a phased array antenna system with controllable electrical tilt. The antenna system is suitable for use in many telecommunications systems, but finds particular application in cellular mobile radio networks, commonly referred to as mobile telephone networks. More specifically, but without limitation, the antenna system of the invention may be used with second generation (2G) mobile telephone networks such as the GSM system, CDMA (IS95), D-AMPS (IS136) and PCS systems and third generation (3G) mobile telephone networks such as the Universal Mobile Telephone System (UMTS), and other cellular systems.
(2) Description of the Art
Operators of cellular mobile radio networks generally employ their own base-stations, each of which has at least one antenna. In a cellular mobile radio network, the antennas are a primary factor in defining a coverage area in which communication to the base station can take place. The coverage area is generally divided into a number of cells, each associated with a respective antenna and base station.
Each cell contains a base station for radio communication with all of the mobile radios (mobiles) in that cell. Base stations are interconnected by other means of communication, usually fixed land-lines, or point-to-point radio links, allowing mobile radios throughout the cell coverage area to communicate with each other as well as with the public telephone network outside the cellular mobile radio network.
Cellular mobile radio networks which use phased array antennas are known: such an antenna comprises an array (usually eight or more) individual antenna elements such as dipoles or patches. The antenna has a radiation pattern incorporating a main lobe and sidelobes. The centre of the main lobe is the antenna's direction of maximum sensitivity in reception mode and the direction of its main output radiation beam in transmission mode. It is a well-known property of a phased array antenna that if signals received by antenna elements are delayed by a delay which varies with element distance from an edge of the array, then the antenna main radiation beam is steered towards the direction of increasing delay. The angle between main radiation beam centres corresponding to zero and non-zero variation in delay, i.e. the angle of tilt, depends on the rate of change of delay with distance across the array.
Delay may be implemented equivalently by changing signal phase, hence the expression phased array. The main beam of the antenna pattern can therefore be altered by adjusting the phase relationship between signals fed to antenna elements. This allows the beam to be steered to modify the coverage area of the antenna.
Operators of phased array antennas in cellular mobile radio networks have a requirement to adjust their antennas' vertical radiation pattern, i.e. the pattern's cross-section in the vertical plane. This is necessary to alter the vertical angle of the antenna's main beam, also known as the “tilt”, in order to adjust the coverage area of the antenna. Such adjustment may be required, for example, to compensate for change in cellular network structure or number of base stations or antennas. Adjustment of antenna angle of tilt is known both mechanically and electrically, either individually or in combination.
Antenna angle of tilt may be adjusted mechanically by moving antenna elements or their housing (radome): it is referred to as adjusting the angle of “mechanical tilt”. As described earlier, antenna angle of tilt may be adjusted electrically by changing time delay or phase of signals fed to or received from each antenna array element (or group of elements) without physical movement: this is referred to as adjusting the angle of “electrical tilt”. When used in a cellular mobile radio network, a phased array antenna's vertical radiation pattern (VRP) has a number of significant requirements:                1. high boresight gain;        2. a first upper side lobe level sufficiently low to avoid interference to mobiles using a base station in a different cell;        3. a first lower side lobe level sufficiently high to allow communications in the immediate vicinity of the antenna;        4. side lobe levels that remain within predetermined limits when the antenna is electrically tilted.        
The requirements are mutually conflicting, for example, increasing the boresight gain may increase the level of the side lobes. Also, the direction and level of the side lobes may change when the antenna is electrically tilted.
A first upper side lobe maximum level, relative to the boresight level, of −18 dB has been found to provide a convenient compromise in overall system performance.
The effect of adjusting either the angle of mechanical tilt or the angle of electrical tilt is to reposition the boresight so that, for an array lying in a vertical plane, it points either above r below the horizontal plane, and hence changes the coverage area of the antenna. It is desirable to be able to vary both the mechanical tilt and the electrical tilt of a cellular radio base station's antenna: this allows maximum flexibility in optimisation of cell coverage, since these forms of tilt have different effects on antenna ground coverage and also on other antennas in the station's immediate vicinity. Also, operational efficiency is improved if the angle of electrical tilt can be adjusted remotely from the antenna assembly. Whereas an antenna's angle of mechanical tilt may be adjusted by re-positioning its radome, changing its angle of electrical tilt requires additional electronic circuitry which increases antenna cost and complexity. Furthermore, if a single antenna is shared between a number of operators it is preferable to provide a different angle of electrical tilt for each operator.
The need for an individual angle of electrical tilt from a shared antenna has hitherto resulted in compromises in the performance of the antenna. The boresight gain will decrease in proportion to the cosine of the angle of tilt due to a reduction in the effective aperture of the antenna (this is unavoidable and happens in all antenna designs). Further reductions in boresight gain may result as a consequence of the method used to change the angle of tilt.
R. C. Johnson, Antenna Engineers Handbook, 3rd Ed 1993, McGraw Hill, ISBN 0-07-032381-X, Ch 20, FIG. 20-2 discloses a known method for locally or remotely adjusting a phased array antenna's angle of electrical tilt. In this method a radio frequency (RF) transmitter carrier signal is fed to the antenna and distributed to the antenna's radiating elements. Each antenna element has a respective phase shifter associated with it so that signal phase can be adjusted as a function of distance across the antenna to vary the antenna's angle of electrical tilt. The distribution of power to antenna elements when the antenna is not tilted is proportioned so as to set the side lobe level and boresight gain. Optimum control of the angle of tilt is obtained when the phase front is controlled for all angles of tilt so that the side lobe level is not increased over the tilt range. The angle of electrical tilt can be adjusted remotely, if required, by using a servo-mechanism to control the phase shifters.
This prior art method antenna has a number of disadvantages. A phase shifter is required for every antenna element. The cost of the antenna is high due to the number of phase shifters required. Cost reduction by applying delay devices to respective groups of antenna elements instead of to individual elements increases the side lobe level. Mechanical coupling of delay devices is used to adjust delays, but it is difficult to do this correctly; moreover, mechanical links and gears are required resulting in a non-optimum distribution of delays. The upper side lobe level increases when the antenna is tilted downwards thus causing a potential source of interference to mobiles using other cells. If the antenna is shared by a number of operators, the operators have a common angle of electrical tilt instead of different angles. Finally, if the antenna is used in a communications system having (as is common) up-link and down-link at different frequencies (frequency division duplex system), the angles of electrical tilt in transmit and receive modes are different.
Patent Application Nos. PCT/GB2002/004166, PCT/GB2002/004930, GB0307558.7 and GB0311371.9 describe different approaches to locally or remotely adjusting an antenna's angle of electrical tilt by means of a phase difference between two signals fed to antenna circuitry. PCT/GB2004/001297 relates to adjusting electrical tilt by dividing a carrier signal into two signals, variably phase shifting one signal relative to the other and applying a phase to power conversion to the resulting signals. The converted signals are split and subjected to power to phase conversion for supply to antenna elements. Electrical tilt is adjusted by varying the phase shift between the two signals. PCT/GB2004/002016 also relates to introducing a variable relative phase shift between two signals, which are then split into components: vectorial combinations of the components are formed to provide respective drive signals for individual antenna elements. Here again electrical tilt is adjusted by varying the phase shift between the two signals.
There is however a problem concerned with splitting RF signals, in that splitter ratios can be too high to be implemented in a single splitting operation: it may require two or more cascaded operations which increases circuit size, cost and complexity. The reason for this lies in the fact that splitters are implemented by dividing a microstrip track on a circuit board into narrower strips with different impedance compared to the track before division. Microstrip impedance is related to track width by a highly complicated and empirical expression, but for a typical board substrate thickness a 50 Ohm track would be 2.8 mm wide. The track narrows as the impedance is increased until it is too narrow for a reliable bond to the substrate Failure to produce a reliable bond occurs at track widths below about 0.2 mm: this width gives an impedance of about 150 Ohms, representing a splitter ratio of 9.5 dB, which it is therefore desirable not to exceed for a single splitter.
PCT/GB2004/001297 requires splitter ratios of 19 dB, which means cascading at least two splitter operations.
Other potential problems are as follows: a) too many splitter outputs may be required than can be implemented in a single splitter; b) widely varying splitter ratios reduce the frequency range over which an antenna can tilt while retaining a desirable low side lobe level; and c) multiple splitters result in a corporate signal feed network to an antenna with different feeder lengths to individual antenna elements. Of these c) requires additional components to be inserted so that the signal transit time to each element is the same to obtain a phase neutral network and an optimised frequency response. All of these problems make it desirable to reduce the number of splitters and the splitter ratios.